Liquid-crystalline coating fluid and polarizing film

ABSTRACT

A liquid-crystalline coating fluid which comprises: an azo compound represented by the following general formula (1); and a solvent to dissolve the azo compound: 
     
       
         
         
             
             
         
       
     
     wherein R is an amino group, a cyano group, a hydroxyl group, a nitro group, a halogen group or a hydrogen atom; and M is a counterion.

CROSS-REFERENCE OF RELATED APPLICATION

This application is a Divisional of application Ser. No. 13/505,592,filed on May 2, 2012, which is a 371 of International Application No.PCT/JP2010/052398, filed Feb. 18, 2010, which claims priority ofJapanese Patent Application No. 2009-256198, the entire contents ofwhich are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a liquid-crystalline coating fluid anda polarizing film made from it.

BACKGROUND OF THE INVENTION

A polarizing plate absorbs linear polarization in an absorption axisdirection and allows the linear polarization in a transmission axisdirection to pass through. Conventionally, a polarizing plate obtainedby dying a resin film, such as a polyvinyl alcohol or the like withiodine or a dichromatic dye and stretching the film in one direction hasbeen widely used. However, there has been a problem that theaforementioned polarizing plate is poor in heat resistance and lightresistance depending on the kind of the dye or the resin film. Further,there has been a drawback that the polarizing plate has a relativelygreat thickness.

In contrast, a method for forming a polarizing film by casting aliquid-crystalline coating fluid containing a liquid-crystallinecompound on a substrate, such as a glass plate or a resin film and thelike to orient the liquid-crystalline compound is known. A lyotropicliquid-crystalline compound is suitable as a liquid crystalline-compoundto be used for this purpose.

The lyotropic liquid-crystalline compound forms supramolecularaggregates exhibiting liquid crystallinity in the solution, so that thelong axis direction of the supramolecular aggregates is oriented in acasting direction when casting with the application of a shearing stressto the liquid-crystalline coating fluid containing this.

Lyotropic liquid-crystalline compound-based polarizing films do not needto be stretched and are easy to have a greater width. Further, it ispossible to reduce the thickness of the lyotropic liquid-crystallinecompound-based polarizing films significantly.

Azo compounds having hydroxyl groups in ortho positions of naphthalenerings coupled to azo groups are known as lyotropic liquid-crystallinecompounds suitable for making a polarizing film (JP 2007-126628 A).

The polarizing film obtained by casting a liquid-crystalline coatingfluid including such an azo compound exhibits absorption dichroism in awavelength region having wide visible light. However, the polarizingfilm may have a low dichroic ratio (e.g., the dichroic ratio is lessthan 10) depending on the molecular structure of the azo compound.

SUMMARY OF THE INVENTION

A polarizing film obtained by casting a liquid-crystalline coating fluidincluding an azo compound exhibits absorption dichroism in a wavelengthregion having wide visible light. However, the polarizing film may havea low dichroic ratio (e.g., the dichroic ratio is less than 10)depending on the molecular structure of the azo compound.

It is an object of the present invention to stably obtain a polarizingfilm having a high dichroic ratio using a liquid-crystalline coatingfluid including a novel azo compound.

The summary of the present invention is described as below.

In a first preferred aspect, a liquid-crystalline coating fluidaccording to the present invention comprises: an azo compoundrepresented by the following general formula (1); and a solvent todissolve the azo compound:

wherein R is an amino group, a cyano group, a hydroxyl group, a nitrogroup, a halogen group or a hydrogen atom; and M is a counterion.

In a second preferred aspect, a liquid-crystalline coating fluidaccording to the present invention comprises: an azo compoundrepresented by the following general formula (2); and a solvent todissolve the azo compound:

wherein R is an amino group, a cyano group, a hydroxyl group, a nitrogroup, a halogen group or a hydrogen atom; and M is a counterion.

In a third preferred aspect, a liquid-crystalline coating fluidaccording to the present invention comprises: an azo compoundrepresented by the general formula (6); and a solvent to dissolve theazo compound.

In a fourth preferred aspect, a polarizing film according to the presentinvention is obtained by casting the aforementioned liquid-crystallinecoating fluid in a thin film state.

ADVANTAGE OF THE INVENTION

Inventors of the present invention synthesized a novel azo compound inwhich a hydrazone structure was not formed near a sulfonic acid group. Apolarizing film manufactured by casting this azo compound stablyexhibits a high dichroic ratio.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Since an azo compound having a hydroxyl group in an ortho-position of anaphthalene ring bonded to an azo group is capable of forming an azostructure represented by the following chemical formula (3) and ahydrazone structure represented by the following chemical formula (4),the azo compound is presumed to exhibit absorption dichroism in awavelength region with wide visible light:

According to the study of the inventors of the present invention, aconventional azo compound has a hydrazone structure near a bulkysulfonic acid group, which could lead to damage flatness of thehydrazone structure. This decreases the dichroic ratio. Accordingly, apolarizing film manufactured using a conventional azo compound does nothave a stable dichroic ratio.

To avoid this, the inventors of the present invention synthesized anovel azo compound in which a hydrazone structure was not formed near asulfonic acid group. A polarizing film manufactured by using this azocompound stably exhibits a high dichroic ratio.

[Liquid-Crystalline Coating Fluid]

A liquid-crystalline coating fluid of the present invention comprises:an azo compound represented by the following general formula (1); and asolvent to dissolve the azo compound.

In the general formula (1), R is an amino group, a cyano group, ahydroxyl group, a nitro group, a halogen group or a hydrogen atom; and Mrepresents a counterion and is preferably a hydrogen atom, an alkalinemetal atom, an alkaline-earth metal atom, metal ions or substituted orunsubstituted ammonium ions.

Examples of a metal ion include, for instance, Na⁺, Li⁺, Ni²⁺, Fe³⁺,Cu²⁺, Ag⁺, Zn²⁺, Al³⁺, Pd²⁺, Cd²⁺, Sn²⁺, Co²⁺, Mn²⁺, or Ce³⁺ and thelike. When the counterion M is a multivalent ion, a plurality of azocompounds share one multivalent ion (counterion).

In the liquid-crystalline coating fluid, the azo compound formsaggregates in the solution and exhibits a liquid crystal phase. Theliquid crystal phase is typically a nematic liquid crystal phase, ahexagonal liquid crystal phase or the like.

The azo compound in the liquid-crystalline coating fluid preferably hasa concentration of 0.5% to 50% by weight. The liquid-crystalline coatingfluid exhibits a stable liquid crystal phase at least a portion of thisconcentration range.

The liquid-crystalline coating fluid of the present invention maycontain anything including the aforementioned azo compound and asolvent, for instance, the liquid-crystalline coating fluid of thepresent invention may contain other liquid-crystalline compound or anyadditives. Examples of the additives include a surfactant, anantioxidant, an antistatic agent and the like. The concentration of theadditives is generally less than 10% by weight.

[Azo Compound]

The azo compound to be used in the present invention is a compoundrepresented by the aforementioned general formula (1). The azo compoundto be used in the present invention is preferably a compound representedby the general formula (2) mentioned below. In the general formula (2)mentioned below, R and M are the same as those in the aforementionedgeneral formula (1).

The azo compound represented by the general formula (1) or (2) forms ahydrazone structure having flatness higher than the conventional ones ina position away from the sulfonic acid group. A polarizing film obtainedusing this azo compound stably exhibits a high dichroic ratio becausethe hydrazone structure has high flatness.

As shown in the following reaction formula (5), in accordance with aconventional method, a monoazo compound is obtained by diazotizing andcoupling two kinds of compounds typically having an amino group and isfurther subject to diazotization and coupling reaction with a hydroxylnaphthalene sulfonic acid derivative to obtain an azo compoundrepresented by the general formula (1) or (2). The conventional methodis typically described in “Riron Seizou Senryokagaku” Fifth Edition(Theoretical Production Dye Chemistry) Yutaka Hosoda (published on Jul.15, 1968, GIHODO SHUPPAN Co., Ltd.), pages 135 to 152).

[Solvent]

Solvents to be used in the present invention dissolve the aforementionedazo compounds and hydrophilic solvents are preferably used as solvents.The hydrophilic solvents are preferably water, alcohol kinds, cellosolvekinds and mixture of thereof. Water-soluble compounds, such as glycerin,ethyleneglycol or the like may be added to the solvents. These additivescan be used to control readily solubility of the azo compound and thedrying rate of the liquid-crystalline coating fluid.

[Polarizing Film]

A polarizing film of the present invention can be obtained by castingthe aforementioned liquid-crystalline coating fluid.

The polarizing film of the present invention preferably exhibitsabsorption dichroism in a visible light region (at a wavelength of 380nm to 780 nm). Such characteristics are obtained by the orientation ofthe aforementioned azo compound in the polarizing film.

The aforementioned azo compound forms supramolecular aggregates in theliquid-crystalline coating fluid. Accordingly, the long axis directionof the supramolecular aggregates is oriented in the flowing direction bycasting the liquid-crystalline coating fluid while applying a shearingforce to the liquid-crystalline coating fluid to flow. In addition tothe shearing force, an orientation means may combine orientationtreatment, such as rubbing treatment and optical orientation or the likeand orientation by a magnetic field and an electric field.

The polarizing film of the present invention preferably has a thicknessof 0.1 μm to 5 μm. The polarizing film of the present inventionpreferably has a dichroic ratio of 10 or higher.

While it is to be understood that the flow casting means of theliquid-crystalline coating fluid is not particularly limited, if onlythe liquid-crystalline coating fluid is uniformly cast. An appropriatecoater is used.

While it is to be understood that a substrate for casting theliquid-crystalline coating fluid of the present invention is notparticularly limited, a single layer or a plurality of laminates (forexample, including an orientation film) may be used. Examples of thespecific substrate include a glass plate and a resin film. Although thethickness of the substrate is not particularly limited except for itsapplication, the thickness is generally in the range between 1 μm to1,000 μm.

When the substrate includes an orientation film, the orientation film ispreferably treated with orientation. Examples of the substrate includingan orientation film include a substrate made by coating a polyimide filmon a glass plate. Orientation property is given by a known method, forexample, by mechanical orientation treatment, such as rubbing or thelike and optical orientation treatment or the like.

An alkali-free glass to be used for a liquid crystal cell is preferablyused as a substrate glass. A resin film substrate is preferably used forthe uses that require flexibility. The surface of the resin film may beorientation treated by rubbing or the like. Alternatively, anorientation film composed of other materials may be formed on thesurface of the resin film.

While materials of the resin film used for the substrate are notparticularly limited, only if the materials are resins having filmforming properties, examples of the materials include stylene resins,(meta) acrylic acid resins, polyester resins, polyolefin resins,norbornene resins, polyimide resins, cellulose resins, polyvinyl alcoholresins, and polycarbonate resins or the like.

The casted liquid-crystalline coating fluid may be dried when necessary.While the drying method is not particularly limited, natural drying,reduced-pressure drying, drying by heating, and drying by heating underreduced pressure or the like may be used.

Any drying methods using a drying apparatus, such as an aircirculation-type drying oven or heated rolls and the like are used asdrying by heating means. The drying temperature in the case of drying byheating is preferably 50° C. to 120° C.

EXAMPLES

The present invention will be more clearly understood by referring tothe Examples below. However, the Examples should not be construed tolimit the invention in any way.

Example 1

A monoazo compound was obtained by diazonium salification of3-amino-2,7-naphthalene disulfonic acid using sodium nitrite andhydrochloric acid and followed by coupling reaction with5-amino-2-naphthalene sulfonic acid in an acidic cool temperatureaqueous solution.

This monoazo compound was subject to diazonuim salification using sodiumnitrite and hydrochloric acid and then was subject to coupling reactionwith 5-amino-2-naphthol in a weakly basic cool water solution and wasfurther subject to conversion into lithium salt from sulfonic acid saltto obtain an azo compound with the following structural formula (6).

The aforementioned azo compound was dissolved in ion-exchange water andwas adjusted so that the concentration of the azo compound might be 30%by weight. As a result, a liquid-crystalline coating fluid exhibiting anematic liquid crystal phase was obtained. Ion-exchange water wasfurther added to the liquid-crystalline coating fluid to adjust that theconcentration of the azo compound might be 5% by weight to obtain acoating fluid.

The aforementioned coating fluid was cast by flowing on a surface of anorbornene polymer film (produced by Nippon Zeon Co., Ltd., product name“Zeonor”) with rubbing treatment and corona treatment using a bar coater(produced by BUSCHMAN, product name “Mayer rot HS4”) to obtain apolarizing film with a thickness of 0.4 μm by natural drying in atemperature-controlled room at 23° C. Table 1 shows a dichroic ratio ofthe obtained polarizing film.

TABLE 1 Chemical structural formula Dichroic ratio Example 1

(6) 30 Example 2

(7) 18 Example 3

(8) 14 Example 4

(9) 13 Example 5

(10) 12

Example 2

The azo compound of the following structural formula (7) was obtained bythe method in the same manner as in Example 1 except for using6-hydroxy-2-naphthonitrile instead of 5-amino-2-naphthol in Example 1.

The aforementioned azo compound was dissolved in ion-exchange water toadjust so that the concentration of the azo compound might be 30% byweight. As a result, a liquid-crystalline coating fluid exhibiting anematic liquid crystal phase was obtained. Ion-exchange water wasfurther added to the liquid-crystalline coating fluid to adjust so thatthe concentration of the azo compound might be 5% by weight to obtain acoating fluid.

The aforementioned coating fluid was applied to the surface of anorbornene polymer film (produced by Nippon Zeon Co., Ltd., product name“Zeonor”) with rubbing treatment and corona treatment using a bar coater(produced by BUSCHMAN, product name “Mayer rot HS4”) to obtain apolarizing film with a thickness of 0.4 μm by natural drying in atemperature-controlled room at 23° C. Table 1 shows a dichroic ratio ofthe obtained polarizing film.

Example 3

An azo compound of the following structural formula (8) was obtained inthe same manner as in Example 1 except for using8-amino-2-naphthalenesulfonic acid in place of5-amino-2-naphthalenesulfonic acid and using 8-amino-2-naphthol in placeof 5-amino-2-naphthol.

The aforementioned azo compound was dissolved in ion-exchange water toadjust so that the concentration of the azo compound might be 30% byweight. As a result, a liquid-crystalline coating fluid exhibiting anematic liquid crystal phase was obtained. Ion-exchange water wasfurther added to the liquid-crystalline coating fluid to adjust so thatthe concentration of the azo compound might be 5% by weight to obtain acoating fluid.

The aforementioned coating fluid was applied to the surface of anorbornene polymer film (produced by Nippon Zeon Co., Ltd., product name“Zeonor”) with rubbing treatment and corona treatment using a bar coater(produced by BUSCHMAN, product name “Mayer rot HS4”) to obtain apolarizing film with a thickness of 0.4 μm by natural drying in atemperature-controlled room at 23° C. Table 1 shows a dichroic ratio ofthe obtained polarizing film.

Example 4

An azo compound of the following structural formula (9) was obtained bythe method in the same manner as in Example 1 except for using8-amino-2-naphthalenesulfonic acid in place of5-amino-2-naphthalenesulfonic acid and using6-amino-1,3-naphthtalenedisulfonic acid in place of3-amino-2,7-naphthalenedisulfonic acid.

The aforementioned azo compound was dissolved in ion-exchange water toadjust so that the concentration of the azo compound might be 30% byweight. As a result, a liquid-crystalline coating fluid exhibiting anematic liquid crystal phase was obtained. Ion-exchange water wasfurther added to the liquid-crystalline coating fluid to adjust so thatthe concentration of the azo compound might be 5% by weight to obtain acoating fluid.

The aforementioned coating fluid was applied to the surface of anorbornene polymer film (produced by Nippon Zeon Co., Ltd., product name“Zeonor”) with rubbing treatment and corona treatment using a bar coater(produced by BUSCHMAN, product name “Mayer rot HS4”) to obtain apolarizing film with a thickness of 0.4 μm by natural drying in atemperature-controlled room at 23° C. Table 1 shows a dichroic ratio ofthe obtained polarizing film.

Example 5

An azo compound of the following structural formula (10) was obtained inthe same manner as in Example 1 except for using8-amino-2-naphthalenesulfonic acid in place of5-amino-2-naphthalenesulfonic acid.

The aforementioned azo compound was dissolved in ion-exchange water toadjust so that the concentration of the azo compound might be 30% byweight. As a result, a liquid-crystalline coating fluid exhibiting anematic liquid crystal phase was obtained. Ion-exchange water wasfurther added to the liquid-crystalline coating fluid to adjust so thatthe concentration of the azo compound might be 5% by weight to obtain acoating fluid.

The aforementioned coating fluid was applied to the surface of anorbornene polymer film (produced by Nippon Zeon Co., Ltd., product name“Zeonor”) with rubbing treatment and corona treatment using a bar coater(produced by BUSCHMAN, product name “Mayer rot HS4”) to obtain apolarizing film with a thickness of 0.4 μm by natural drying in atemperature-controlled room at 23° C. Table 1 shows a dichroic ratio ofthe obtained polarizing film.

[Measurement Method] [Observation of Liquid Crystal Phase]

A small quantity of the coating fluid was sandwiched by two pieces ofslide glasses to observe using a polarization microscope (produced byOlympus, product name: “OPTIPHOT-POL”) with a large-size sample heatingand cooling stage (produced by JAPAN HIGH TECH CO., LTD., product name:“10013L”).

[Measurement of Dichroic Ratio]

Measuring light of linear polarization (produced by JASC Corporation;product name: V-7100) was allowed to enter using a spectrophotometerwith Glan-Thompson polarizer. And k₁ and k₂ of Y value whose visibilityhad been corrected were obtained to calculate a dichroic ratio from thefollowing equation:

Dichroic ratio=log(1/k ₂)/log(1/k ₁)

wherein k₁ is a transmittance of a linear polarization in a maximumtransmittance direction and k₂ is a transmittance of a linearpolarization in a direction that is perpendicular to the maximumtransmittance direction.

INDUSTRIAL APPLICABILITY

The polarizing film of the present invention is preferably used as apolarizing element. A polarizing element is preferably used for liquidcrystal panels for a variety of devices, such as liquid crystal panels,such as liquid crystal television units, computer displays, mobilephones, digital cameras, video cameras, portable game devices, and carnavigation systems or the like.

The polarizing film of the present invention may be released from thesubstrate. Alternatively, the polarizing film may be used whileremaining laminated on the substrate. In the case where the polarizingfilm is used for optical uses while remaining laminated on thesubstrate, the substrate is preferably transparent to visible light. Inthe case where the polarizing film is released from the substrate, thepolarizing film is preferably used while being laminated on othersupport or optical elements.

What is claimed is:
 1. A polarizing film obtained by casting aliquid-crystalline coating fluid in a thin film state, theliquid-crystalline coating fluid comprising: an azo compound representedby the following general formula (1); and a solvent to dissolve the azocompound:

wherein R is an amino group, a cyano group, a hydroxyl group, a nitrogroup, a halogen group or a hydrogen atom; and M is a counterion.